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Posted on 2014-09-29 21:31 鑫龍 閱讀(7732) 評論(0) 編輯 收藏 引用 所屬分類: linux內核
轉自： http://my.oschina.net/moooofly/blog/175019最近看 GLib 的代碼遇到這個東東，網上搜索一圈，發現很多人都寫過這個，自己今天才研究到，汗顏一下，掃盲一個點，留此記錄為證！ 首先看一篇最官方的講解： ====== likely() and unlikely() What are they ? In Linux kernel code, one often find calls to likely() and unlikely(), in conditions, like :
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bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx);
if (unlikely(!bvl)) {
mempool_free(bio, bio_pool);
bio = NULL;
goto out;
}
| In fact, these functions are hints for the compiler that allows it to correctly optimize the branch, by knowing which is the likeliest one. The definitions of these macros, found in include/linux/compiler.h are the following :
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#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)
| The GCC documentation explains the role of __builtin_expect() :
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-- Built-in Function: long __builtin_expect (long EXP, long C)
You may use `__builtin_expect' to provide the compiler with branch
prediction information. In general, you should prefer to use
actual profile feedback for this (`-fprofile-arcs'), as
programmers are notoriously bad at predicting how their programs
actually perform. However, there are applications in which this
data is hard to collect.
The return value is the value of EXP, which should be an integral
expression. The value of C must be a compile-time constant. The
semantics of the built-in are that it is expected that EXP == C.
For example:
if (__builtin_expect (x, 0))
foo ();
would indicate that we do not expect to call `foo', since we
expect `x' to be zero. Since you are limited to integral
expressions for EXP, you should use constructions such as
if (__builtin_expect (ptr != NULL, 1))
error ();
when testing pointer or floating-point values.
| How does it optimize things ? It optimizes things by ordering the generated assembly code correctly, to optimize the usage of the processor pipeline. To do so, they arrange the code so that the likeliest branch is executed without performing any jmp instruction (which has the bad effect of flushing the processor pipeline). To see how it works, let's compile the following simple C user space program with gcc -O2 :
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int main(char *argv[], int argc)
{
int a;
/* Get the value from somewhere GCC can't optimize */
a = atoi (argv[1]);
if (unlikely (a == 2))
a++;
else
a--;
printf ("%d\n", a);
return 0;
}
| Now, disassemble the resulting binary using objdump -S (comments added by me) :
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080483b0 <main>:
// Prologue
80483b0: 55 push %ebp
80483b1: 89 e5 mov %esp,%ebp
80483b3: 50 push %eax
80483b4: 50 push %eax
80483b5: 83 e4 f0 and $0xfffffff0,%esp
// Call atoi()
80483b8: 8b 45 08 mov 0x8(%ebp),%eax
80483bb: 83 ec 1c sub $0x1c,%esp
80483be: 8b 48 04 mov 0x4(%eax),%ecx
80483c1: 51 push %ecx
80483c2: e8 1d ff ff ff call 80482e4 <atoi@plt>
80483c7: 83 c4 10 add $0x10,%esp
// Test the value
80483ca: 83 f8 02 cmp $0x2,%eax
// --------------------------------------------------------
// If 'a' equal to 2 (which is unlikely), then jump,
// otherwise continue directly, without jump, so that it
// doesn't flush the pipeline.
// --------------------------------------------------------
80483cd: 74 12 je 80483e1 <main+0x31>
80483cf: 48 dec %eax
// Call printf
80483d0: 52 push %edx
80483d1: 52 push %edx
80483d2: 50 push %eax
80483d3: 68 c8 84 04 08 push $0x80484c8
80483d8: e8 f7 fe ff ff call 80482d4 <printf@plt>
// Return 0 and go out.
80483dd: 31 c0 xor %eax,%eax
80483df: c9 leave
80483e0: c3 ret
| Now, in the previous program, replace the unlikely() by a likely(), recompile it, and disassemble it again (again, comments added by me) :
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080483b0 <main>:
// Prologue
80483b0: 55 push %ebp
80483b1: 89 e5 mov %esp,%ebp
80483b3: 50 push %eax
80483b4: 50 push %eax
80483b5: 83 e4 f0 and $0xfffffff0,%esp
// Call atoi()
80483b8: 8b 45 08 mov 0x8(%ebp),%eax
80483bb: 83 ec 1c sub $0x1c,%esp
80483be: 8b 48 04 mov 0x4(%eax),%ecx
80483c1: 51 push %ecx
80483c2: e8 1d ff ff ff call 80482e4 <atoi@plt>
80483c7: 83 c4 10 add $0x10,%esp
// --------------------------------------------------
// If 'a' equal 2 (which is likely), we will continue
// without branching, so without flusing the pipeline. The
// jump only occurs when a != 2, which is unlikely.
// ---------------------------------------------------
80483ca: 83 f8 02 cmp $0x2,%eax
80483cd: 75 13 jne 80483e2 <main+0x32>
// Here the a++ incrementation has been optimized by gcc
80483cf: b0 03 mov $0x3,%al
// Call printf()
80483d1: 52 push %edx
80483d2: 52 push %edx
80483d3: 50 push %eax
80483d4: 68 c8 84 04 08 push $0x80484c8
80483d9: e8 f6 fe ff ff call 80482d4 <printf@plt>
// Return 0 and go out.
80483de: 31 c0 xor %eax,%eax
80483e0: c9 leave
80483e1: c3 ret
| How should I use it ? You should use it only in cases when the likeliest branch is very very very likely, or when the unlikeliest branch is very very very unlikely. ====== 看完最權威的，下面看下“民間”的說法：
====== likely,unlikely宏與GCC內建函數__builtin_expect() 在 GCC 手冊中對 __builtin_expect() 的描述是這樣的： 由于大部分程序員在分支預測方面做得很糟糕，所以 GCC 提供了這個內建函數來幫助程序員處理分支預測，優化程序。其第一個參數 exp 為一個整型表達式，這個內建函數的返回值也是這個 exp ，而 c 為一個編譯期常量。這個函數的語義是：你期望 exp 表達式的值等于常量 c ，從而 GCC 為你優化程序，將符合這個條件的分支放在合適的地方。一般情況下，你也許會更喜歡使用 gcc 的一個參數 '-fprofile-arcs' 來收集程序運行的關于執行流程和分支走向的實際反饋信息。 因為這個程序只提供了整型表達式，所以如果你要優化其他類型的表達式，可以采用指針的形式。 likely 和 unlikely 是 gcc 擴展的跟處理器相關的宏：
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#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)
| 現在處理器都是流水線的，有些里面有多個邏輯運算單元，系統可以提前取多條指令進行并行處理，但遇到跳轉時，則需要重新取指令，這相對于不用重新去指令就降低了速度。 所以就引入了 likely 和 unlikely ，目的是增加條件分支預測的準確性，cpu 會提前裝載后面的指令，遇到條件轉移指令時會提前預測并裝載某個分 支的指令。unlikely 表示你可以確認該條件是極少發生的，相反 likely 表示該條件多數情況下會發生。編譯器會產生相應的代碼來優化 cpu 執行效率。 因此程序員在編寫代碼時可以根據判斷條件發生的概率來優化處理器的取指操作。 例如：
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int x, y;
if(unlikely(x > 0))
y = 1;
else
y = -1;
| 上面的代碼中 gcc 編譯的指令會預先讀取 y = -1 這條指令，這適合 x 的值大于 0 的概率比較小的情況。 如果 x 的值在大部分情況下是大于 0 的，就應該用 likely(x > 0)，這樣編譯出的指令是預先讀取 y = 1 這條指令了。這樣系統在運行時就會減少重新取指了。
====== 內核中的 likely() 與 unlikely() 首先要明確：
- if(likely(value)) 等價于 if(value)
- if(unlikely(value)) 也等價于 if(value)
__builtin_expect() 是 GCC (version >= 2.96）提供給程序員使用的，目的是將“分支轉移”的信息提供給編譯器，這樣編譯器可以對代碼進行優化，以減少指令跳轉帶來的性能下降。 __builtin_expect((x),1) 表示 x 的值為真的可能性更大； __builtin_expect((x),0) 表示 x 的值為假的可能性更大。 也就是說，使用 likely() ，執行 if 后面的語句 的機會更大，使用 unlikely()，執行 else 后面的語句的機會更大。通過這種方式，編譯器在編譯過程中，會將可能性更大的代碼緊跟著起面的代碼，從而減少指令跳轉帶來的性能上的下降。 ====== 看完一圈別 人寫的東西，自己也要輸出點干貨，列舉 GLib-2.35.4 中 gmacros.h 代碼如下：
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#if defined(__GNUC__) && (__GNUC__ > 2) && defined(__OPTIMIZE__)
#define _G_BOOLEAN_EXPR(expr) \
G_GNUC_EXTENSION ({ \
int _g_boolean_var_; \
if (expr) \
_g_boolean_var_ = 1; \
else \
_g_boolean_var_ = 0; \
_g_boolean_var_; \
})
#define G_LIKELY(expr) (__builtin_expect (_G_BOOLEAN_EXPR(expr), 1))
#define G_UNLIKELY(expr) (__builtin_expect (_G_BOOLEAN_EXPR(expr), 0))
#else
#define G_LIKELY(expr) (expr)
#define G_UNLIKELY(expr) (expr)
#endif
| 由上可以看出， GLib 中使用 _G_BOOLEAN_EXPR(expr) 代替了 !!(expr) 。但功能上是一樣的。
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